1. Field
The following description relates to a passive optical network and, more particularly, to a spectrum-sliced seed light module for a wavelength division multiplexing passive optical network (WDM PON).
2. Description of the Related Art
A dense wavelength division multiplexing passive optical network (WDM PON) gains notice as a next-generation optical network. Above all, in WDM PON technology, an optical transmitter module has to be independent of wavelengths even though it uses multiple optical wavelengths. WDM PON technologies satisfying such requirement are under study worldwide. Wavelength-locking WDM PON and wavelength-reuse WDM PON are currently commercially available WDM PONs.
For wavelength-locking WDM PON, when seed light is injected to a Fabry-Perot laser diode (FP-LD), light with the injected wavelengths is amplified in the FP-LD while light with the other wavelengths is suppressed. In this case, a broadband light source (BLS) is used as a seed light source. In the wavelength-locking scheme, two kinds of BLSs are installed in a central office: one BLS provides seed light to a FP-LD located in an optical line termination (OLT) for the central office; the other provides seed light to a FP-LD incorporated in an optical network unit (ONU) located at the customer premises. Light from the BLS is spectrum-sliced while passing through a WDM MUX incorporated in an OLT and a WDM MUX incorporated in a remote node (RN). The spectrum-sliced seed light is injected to the FP-LD.
For wavelength-reuse WDM PON, a reflective semiconductor optical amplifier (RSOA) is used as a communication light source. When an optical signal including downstream data is sent from an OLT to an ONU, RSOA in the ONU removes the downstream data from the optical signal; the optical signal is converted to pseudo continuous wave (CW) light. The converted light is modulated into upstream data and sent to the OLT for the central office. That is, the modulated optical signal sent from the OLT to the ONU provides seed light to the RSOA incorporated in the ONU.
On the other hand, seed light also needs to be provided to a RSOA incorporated in the OLT. In this case, an external light source is typically used. A BLS is used as an external seed light source. Also in wavelength-reuse scheme, light from the BLS is spectrum-sliced while passing through a WDM MUX incorporated in the OLT. The spectrum-sliced seed light is injected to the RSOA.
Since light from the BLS is spectrum-sliced while passing through WDM MUX on a communication link and is injected to the FP-LD or RSOA, the above-mentioned conventional schemes introduce a loss in optical power during the spectrum-slicing process. As a result, the seed light power needs to be boosted above a predetermined level.